I have deployed Zoiper premium on a couple of devices linked to a phone system for testing. One is android and the other iOS.

I have enabled number re-writing on both devices so phone book entries saved as 07xxxxxxxxx will automatically dial as +447xxxxxxxxx. On both devices this works perfectly, however, when dialling an internal extension number on the phone system or another Zoiper Client, the iOS device prefixes the internal number with +44. This obviously results in an expected 404 error. The Android device just dials the internal number without the prefix.